It is well known that carrier-signal power ratio (CSPR) is a key parameter in direct detection Orthogonal Frequency-Division Multiplexing (OFDM) systems. FIG. 1 shows a graph 100 of a typical relationship between system Bit Error Rate (BER) and CSPR. When CSPR is small, BER is suboptimal because Subcarrier-Signal Beat Interference (SSBI) power is increased relative to signal power. Whereas when CSPR is large, BER is also suboptimal because system noise (usually measured by Optical Signal-to-Noise Ratio (OSNR)) dominates. The sweet spot for CSPR varies with system noise.
For an intensity optical modulator, an optimal CSPR can be set with an optical filter. However, due to laser wavelength wandering, the relative frequency offset between laser wavelength and optical filter center wavelength varies slowly in time, which results in undesirable variation in CSPR and error performance of the system. Therefore, it is desirable to closely monitor and actively control the CSPR.